In the automotive industry in particular, there is a continuous need to lighten vehicles while increasing safety. Thus, several families of steels like the ones mentioned above offering various strength and formability levels have been proposed.
Firstly, steels have been proposed that have micro-alloy elements which hardening is obtained simultaneously by precipitation and by refinement of the grain size. The development of such steels has been followed by the abovementioned Advanced High Strength Steels.
For the purpose of obtaining even higher tensile strength levels, steels exhibiting TRIP behaviour with highly advantageous combinations of properties (tensile strength/formability) have been developed. These properties are associated with the structure of such steels, which consists of a ferritic matrix containing bainite and residual austenite. The residual austenite is stabilized by an addition of silicon or aluminium, these elements retarding the precipitation of carbides in the austenite and in the bainite. The presence of residual austenite gives an un-deformed sheet high ductility.
To achieve an even higher tensile strength, that is to say a level greater than 800-1000 MPa, multiphase steels having a predominantly bainitic structure have been developed. However, the formability and hole expansion properties are insufficient for next generation of automotive parts.
International application WO2009/142362 discloses a cold rolled steel sheet and a hot dip galvanized steel sheet, which has improvement in delayed fracture resistance, a tensile strength of 980 MPa or more and an elongation of 28% or more by adding a suitable amount of Al for raising the stability of retained austenite and resistance against delayed fracture into an optimum composition that can increase the amount of retained austenite. In one or more aspects of this prior art, there are provided a high strength cold rolled steel sheet and a galvanized steel sheet, each of which consists of 0.05 to 0.3 weight percent C, 0.3 to 1.6 weight percent Si, 4.0 to 7.0 weight percent Mn, 0.5 to 2.0 weight percent Al, 0.01 to 0.1 weight percent Cr, 0.02 to 0.1 weight percent Ni and 0.005 to 0.03 weight percent Ti, 5 to 30 ppm B, 0.01 to 0.03 weight percent Sb, 0.008 weight percent or less S, balance Fe and impurities. However such steels are difficult to coat due to high content of alloying elements.
International application WO2012/147898 aims at providing a high-strength steel having excellent hole expansion as well as stability of material properties, and a method for manufacturing the same, the high-strength steel plate having a TS of at least 780 MPa and a TS×EL of at least 22,000 MPa % in a low-C steel composition. The high-strength steel has good formability and stability of material properties has an ingredient composition including, in terms of mass %, 0.03%-0.25% C, 0.4%-2.5% Si, 3.5%-10.0% Mn, 0.1% or less P, 0.01% or less S, 0.01%-2.5% Al, 0.008% or less N, and Si+Al at least 1.0%, the remainder being Fe and unavoidable impurities, the steel structure having, by area ratio, 30%-80% ferrite, 0%-17% martensite, and, by volume ratio, 8% or more of residual austenite, and the average crystalline particle diameter of the residual austenite being 2 μm or less. However such steels are difficult to coat due to high content of alloying elements.
Eventually, application EP2383353 discloses a steel with an elongation at break A80 of minimum 4% and a tensile strength of 900-1500 MPa. It comprises iron and unavoidable impurities and carbon (up to 0.5%), manganese (4-12%), silicon (up to 1%), aluminum (up to 3%), chromium (0.1-4%), copper (up to 2%), nickel (up to 2%), nitrogen (up to 0.05%), phosphorus (up to 0.05%), and sulfur (up to 0.01%), and optionally at most 0.5% of one or more elements comprising vanadium, niobium or titanium. The flat rolled steel product made of the steel, comprises 30-100% of martensite, tempered martensite or bainite and residual quantity of austenite. However, such steel will present low ductility levels leading to poor formability of the steel sheet obtained.